A reciprocating engine has two different movements in the running, which are a piston's back and forth linear motion and flywheel's rotating motion. The crankshaft is an intermediate been used to convert these two movements each other. The engine also needs a complicated timing system working together with pistons and crankshaft to complete four strokes.
The piston has two functions in the engine. First, the piston getting gas power by the one end which works within combustion housing to complete four strokes cycle; second, the another end of piston link to the crankshaft by connecting rod in order to transfer piston's reciprocating motion to flywheel's rotating motion. Each cylinder holds only a combustion housing that piston works within. Usually the multiple cylinders are needed in an engine for increasing the engine power and decreasing vibration.